The present invention relates to an apparatus and a method for diagnosing an abnormality in a variable valve timing apparatus for an internal combustion engine.
There has been known a conventional variable valve timing apparatus for an engine, for changing a rotation phase of a camshaft relative to a crankshaft by controlling a rotation delay of the camshaft relative to the crankshaft based on a friction braking by an electromagnetic brake (Japanese Unexamined Patent Publication 10-153104).
In this variable valve timing apparatus, for example, a basic control amount of the electromagnetic brake is calculated based on a target rotation phase (target rotation delay) and an engine rotation speed, while calculating a feedback control amount from a deviation between the target rotation phase and an actual rotation phase. Then, a final control amount (for example, duty control amount) is determined from the basic control amount and the feedback control amount, to control a current flowing in an electromagnetic coil constituting the electromagnetic brake.
As a variable valve timing apparatus for an internal combustion engine, in general, a hydraulic type apparatus other than the above type is known.
An abnormality diagnosis of such a variable valve timing apparatus (VTC) has been on condition that a target value (hereinafter, target angle) of the rotation phase of the camshaft relative to the crankshaft continues to be held constant for a predetermined period or more (Japanese Unexamined Patent Publication 2000-54870).
In the above variable valve timing apparatus (VTC), since an engine operating region for each target angle is set a relatively large, an opportunity to establish the abnormality diagnosis condition can be secured enough. However, in particular, in the above mentioned electromagnetic brake type apparatus, since the target angle is set fine for each narrow engine operating region, it is hard to sufficiently secure the opportunity to establish the abnormality diagnosis condition, leading a possibility that the diagnosis is not completed.
The present invention has been achieved in view of the foregoing problem, and has an object of securing an opportunity to establish an abnormality diagnosis condition and completing a diagnosis even in a case a target angle is set fine.
To achieve the above object, with the present invention, in a variable valve timing apparatus constituted to change the valve timing by changing a rotation phase of a camshaft relative to a crankshaft, an abnormality diagnosis in this apparatus is performed on condition that a change amount per unit time of a control target value of the rotation phase continues to be held at a predetermined limit value or less for a predetermined period.
Accordingly, even in case the control target value (target angle) of the rotation phase is set fine to be changed by a slight change of an engine operating condition, a diagnosis execution is permitted on condition that the change amount per unit time continues to be held at the limit value or less for the predetermined period.
Therefore, diagnosis opportunity is sufficiently secured and the diagnosis can be completed.
The other objects and features of this invention will become understood from the following description with accompanying drawings.